For we who have been persuaded by many, yea by innumerable, arguments to lead a Christian life, are especially anxious to bring all men as far as possible to receive the whole system of Christian truth; but when we meet with persons who are prejudiced by the calumnies thrown out against Christians, and who, from a notion that Christians are an impious people, will not listen to any who offer to instruct them in the principles of the divine word, then, on the common principles of humanity, we endeavour to the best of our ability to convince them of the doctrine of the punishment of the wicked, and to induce even those who are unwilling to become Christians to accept that truth.
And we are thus anxious to persuade them of the rewards of right living, when we see that many things which we teach about a healthy moral life are also taught by the enemies of our faith. For you will find that they have not entirely lost the common notions of right and wrong, of good and evil. Let all men, therefore, when they look upon the universe, observe the constant revolution of the unerring stars, the converse motion of the planets, the constitution of the atmosphere, and its adaptation to the necessities of the animals, and especially of man, with all the innumerable contrivances for the well-being of mankind; and then, after thus considering the order of the universe, let them beware of doing ought which is displeasing to the Creator of this universe, of the soul and its intelligent principle; and let them rest assured that punishment shall be inflicted on the wicked, and rewards shall be bestowed upon the righteous, by Him who deals with every one as he deserves, and who will proportion His rewards to the good that each has done, and to the account of himself that he is able to give. And let all men know that the good shall be advanced to a higher state, and that the wicked shall be delivered over to sufferings and torments, in punishment of their licentiousness and depravity, their cowardice, timidity, and all their follies.
Source: Against Celsus (New Advent)